Add heat exchangers, turbo expanders, effiecency comes back up.
Large size offsets costs. Standardization on a few sizes also reduces costs.
Go to the thing called the web and look up salt dome and the time constants associated with its size. Note the load leveling capability of such a large volume.
"Waste heat from large dynamic input power being converted to compressed air can be recycled with heat exchanges into constant-near constant output for loads.
Air and oil hybrid can cahnge modes allowing for compressables to be "air blader" for oil.
Dream a little Mr. pessimism or go help clean up the oil disaster because people like you are keeping us now and in the past from becoming more energy independent We now are subsidizing terroism via oil purchases from terorist friendly countries, environmental disasters, and national economic drain (less value/$ ) due to trade deficits.

Compressed air energy storage? Wow. Go back to your physics and thermodynamics classes but before you do, be sure to take of the green colored glasses. You can start building one when you find enough suckers, I mean investors to pay for it.

Now consider a wind turbine site near a salt cavern.
Use the salt cavern as a compressed air storage energy storage device.
Add a hybrid hydraulic-electric-aircompressor.
Any two can be connected or all three. when wind energy exceeds electrical demand, excess energy is used to compress air storage.
When low-no wind present air storage provides electrical base load demand.
Complex system ground based, tower and nacelle low cross section and weight-size-cost.
Cool stuff when do we get started building one?